on , , , 2 minutes reading

For a while a book that had been in my shelf, asking to be read, has been C++ Concurrency in Action, Second Edition by Anthony Williams and published by Manning. Long story short, it is an amazing book, if you are new to concurrency or want to learn more about it (it doesn’t matter you are not into C++), give it a read!.

Anyway, I was writing the examples in C++ and I noticed something interesting with CMake, let me explain with a toy example:

#include <iostream>

int main() {
std::cout << "hello concurrent world!\n";
});
t.join();
}


As you see, something simple, we use the thread header in the standard library introduced in the C++11 standard, let’s build a simple CMakeLists.txt to build with CMake using C++17:

cmake_minimum_required(VERSION 3.15)
project(hello)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)



As usual, we build with the typical cmake and cmake --build . as usual (remember to do this in an out-of-source directory!) and guess what, it builds without any problems! (in Windows and macOS).

But if you try to build the same sample in Linux we have a surprise!

main.cpp:(.text+0xd9): undefined reference to pthread_create'


Wow, an error! it cannot find the pthread library when linking. I was actually expecting something like that but it didn’t fail at all in macOS and Windows, why? because in macOS and Windows you don’t require an external library for threading (so it is included already) while in Linux (and probably FreeBSD) you will need to specify a threading library to use. Knowing this we change the CMakeLists.txt file to this:

cmake_minimum_required(VERSION 3.15)
project(hello)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)


Done, if we try to compile in Linux it will just work! in macOS it will pass the -pthread parameter to LLVM and it will be ignored while in Windows it will just tell you pthreads.h` is not found but CMake will find a Threads library anyway, compiling without problems!